1. Field of the Invention
The invention relates to a process and an apparatus for the hydrogenation of chlorosilanes.
2. Background Art
Processes for the hydrogenation of chlorosilanes at temperatures of >600° C. are known. For example, the hydrogenation of silicon tetrachloride (STC) to trichlorosilane (TCS) has great importance, since large amounts of STC are obtained as by-product in the chemical vapor deposition of semiconductor silicon and STC can be converted back into TCS starting material by means of these processes. Such processes are described, for example, by U.S. Pat. No. 5,422,088 (Burgie et al.), U.S. Pat. No. 3,933,985 (Rogers), U.S. Pat. No. 4,217,334 (Weigert et al.), U.S. Pat. No. 4,536,642 (Hamster et al.) and also by the references cited in these documents.
In all these processes, graphite is used as material of construction for the reactor components, insulation material and heating elements because of its particular mechanical, electrical and chemical properties. U.S. Pat. No. 3,645,686 (Tucker) recognized that the use of graphite electrodes can introduce impurities, e.g. boron, phosphorus, arsenic and antimony, into the semiconductor product. EP 0294047 (McCormick) recognized that contact of graphite with hydrogen at >500° C. can lead to formation of hydrocarbons which leads to the introduction of carbon-containing impurities (methylsilanes) into the product. To avoid the introduction of impurities, it has been proposed that the graphite components be coated with silicon carbide (SiC). The deposition of SiC layers can be effected by means of known CVD processes, for example as described in U.S. Pat. No. 3,459,504 (Bracken) or DE 2379258 (Sirtl).
U.S. Pat. Nos. 4,668,493, 4,702,960, 4,373,006, 4,737,348, and EP 1454670 describe reactors based on SiC-coated carbon material which are used for reactions at high temperatures in the presence of chlorosilanes. With this prior art, as a starting point, DE 4317905 (Burgie et al.) developed an improved reactor for the hydrogenation of chlorosilanes at temperatures of >600° C., having a reaction chamber and heating elements comprising SiC-coated carbon material. A chlorosilane/hydrogen mixture is passed through the reaction chamber. The heating elements are located outside the reaction chamber and do not come into contact with the reaction mixture, the reaction mixture being heated via the walls of the chamber. The heating elements and reactor walls have to reach temperatures of 1600° C. in order to maintain the temperature of 800-1200° C. which is preferred for the hydrogenation of STC in the reaction chamber. This process thus requires an energy input which is higher than that which would be required for the reaction. A higher reactor temperature results in further undesirable effects such as high thermal stress on the components and heating elements, increased corrosion of the components caused by chemical attack by hydrogen, chlorosilanes and HCl, and also deposition of silicon at undesirable places.